The Influence of Mineral NPK Fertiliser Rates on Potassium Dynamics in Soil: Data from a Long-Term Agricultural Plant Fertilisation Experiment

A fertilisation experiment, with the aim to determine the effects of different potassium fertiliser rates and their interactions with nitrogen and phosphorus on field-rotation productivity, potassium balance, fertiliser utilization, and changes in the content of potassium in soil, was carried out in Lithuania between 1971 and 2020. The multi-factorial scheme with 45 treatment plots, where seven rates (including zero) of nitrogen, phosphorus, and potassium fertilisers were studied. The experimental treatments during the study period were carried out on winter wheat, spring wheat, spring barley, sugar beet, spring rapeseed, and annual and perennial grasses. It was found that potassium fertilisers were the most effective on agricultural crops when used in combination with other major plant nutrients—i.e., nitrogen and phosphorus. The required balance of potassium (K₂O) in the soil was measured, when nitrogen and phosphorus fertilisers were applied together to compensate for potassium removal; when applying low nitrogen (N) (72 kg ha⁻¹) and phosphorus (P₂O₅) (64 kg ha⁻¹) fertiliser rates, 128 kg ha⁻¹ of potassium fertilisers are required. When using high nitrogen (180 kg ha⁻¹) and phosphorus (160 kg ha⁻¹) fertiliser rates, 160 kg ha⁻¹ of potassium is needed. The highest potassium uptake, reaching 51.6%, was achieved when plants had been fertilised with nitrogen (108 kg ha⁻¹), phosphorus (96 kg ha⁻¹), and potassium (96 kg ha⁻¹). When fertilising with potassium fertilisers alone, the content of plant-available K₂O content in the soil increased, whereas with fertilisation with nitrogen and phosphorus combined K₂O content is decreased, except in the plots where the plants had been fertilised with potassium fertiliser at rates of 128 kg ha⁻¹ and higher. Due to the influence of fertilisers, the amount of non-exchangeable potassium in the soil also increased, but relatively little compared to the amount of available potassium content. Thus, one of the main conditions for the effective use of potassium fertilisers is ensuring optimal plant nutrition with other nutrition elements, especially nitrogen and phosphorus.